This study is designed to test whether individual differences observed in the response to aerobic exercise-training are associated with heredity. The experiment will be conducted on 12 pairs of adult male monozygotic (MZ) twins. The experimental treatment will last for 100 days and it will be preceeded by two weeks of observation to establish body composition, habitual energy intake, energy cost of habitual activities and other parameters. All twins will be submitted to a negative energy balance of 1000 kcal/day induced by a program of exercise. Treatment will be applied in cycles of 6 consecutive days followed by a day of rest. Changes in body density, total body fat, computerized tomography assessed fat distribution, fat cell size, fat cell lipolysis, and lipoprotein lipase activity from two sites of biopsy (abdominal and femoral), blood lipids and lipoproteins, resting metabolic rate, thermic effect of a meal, exercise tolerance, glucose tolerance and their associated blood glucose, insulin, glucagon, and thyroid hormones variations, the thyrotropes responsiveness to thyrotropin releasing hormone and skeletal muscle eznyme markers will be studied before and after the 100 days of treatment as well as after 25 and 50 days. Diet will be of the mixed type with about 50% carbohydrate, 35% fat, 15% protein. The aerobic exercise program will be of moderate intensity (from 40% to 70% of maximal heart rate reserve) and of sufficient duration to meet the energy cost requirement of the study. This experiment will allow the quantification of the mean response and of the inter-individual differences to negative energy balance generated by exercise-training. Within pair resemblance and between paris variation will also be analysed to establish whether the adaptive changes brought about by the treatment are dependent or independent of the genotype of the individuals. Moreover, the research will make possible the comparison of short term adaptive changes with later adaptations and to test whether the pattern of the response over time is genetically determined. Results of this study should explain some of the unaccounted variations in body composition and metabolism observed during weight reduction.